ORIGIN OF MYCETOCYTES IN PSEUDOCOCCUS. 291 



similar case in Loxa florida where a fusion of cells is likewise un- 

 accompanied by any irregularity in the mitotic spindle formation. 



CONDITIONS IN OLDER EMBRYOS AND IN ADULTS. 



As explained previously, the fusion process is limited chiefly to 

 the period in which the blastoderm is laid down. When the 

 latter is fully established, figures showing apposed nuclei in a 

 single cytoplasmic area become very rare. Most of such figures 

 arise from what is probably an accidental migration of yolk 

 nuclei to the periphery, for a few have been found in the blasto- 

 derm as well as in the giant cell area. It is even doubtful whether 

 in these isolated cases a fusion of nuclei is finally consummated, 

 for no multiple cells have been found in the blastoderm cell region. 

 The increase in the numbers of chromosomes in the giant cells, 

 which is certainly still occurring at this time as well as later, is 

 therefore due principally to a division of chromosomes unaccom- 

 panied by a cytoplasmic division of the cell concerned (Fig. 18). 



When after leaving the periphery the giant cells have become 

 associated with the symbionts, mitotic figures are not so often 

 found in them. Nevertheless they do occur, and successfully as 

 far as the chromosomes are concerned. Cytoplasmic division 

 which before the migration, was undoubtedly completed success- 

 fully in some of the mitoses, is now completed less often. Only 

 in this way can the relatively enormous numbers of chromosomes 

 in the mycetocytes of the adult be explained. Further fusion is in 

 these later stages practically eliminated, since most of the myceto- 

 cytes are almost completely hedged in by the symbiont spheres 

 which they harbor. 



Buchner ('21) estimates the chromosome number in some of 

 the mycetocytes as over 200. In this estimate I can only concur 

 with him. There is in addition to this a decided increase in the 

 individual size of the chromosomes, although this seems to be a 

 variable feature in different mycetocytes (Fig. 19 and 20). 



My material is not favorable for a detailed study of the spindle 

 formation in these later mycetocytes. The centrosomes seem 

 extremely small under even the most favorable circumstances, 

 and special staining methods are difficult to apply to these as to 

 other insect eggs. Apparently mitotic figures are normal now as 



